Nature has a lot to teach us. As part of our special package, we explored how animals, plants, and bacteria use different resilience strategies when faced with scarce resources, predators, and other challenges.
Resilience by regeneration
Humans should envy the axolotl (pictured, above). Our powers of regeneration are limited: Broken bones knit, wounds heal, and large parts of the liver can regenerate, but that's about it. But the axolotl—a large salamander also called the Mexican walking fish because it looks like a 20-centimeter eel with stumpy legs—can replace an entire missing limb or even its tail, which means regrowing the spinal cord, backbone, and muscles. About 30 research teams are probing how these salamanders do it. In the axolotl, they've found, various tissues work together to detect limb loss and coordinate regrowth. In the process, the animals reactivate the same genetic circuits that guided the formation of those structures during embryonic development, causing generalist stem cells to specialize.
Axolotls are only one of several regenerators in the animal kingdom. Flatworms called planarians are even more resilient—able to surge back after losing 90% of their bodies. One small fragment of those 2-centimeter-long aquatic worms can rejuvenate the brain, skin, gut, and all the other functional organs. Again, stem cells are key, and a special set of genes active in muscles tells those stem cells what to do, activating growth and specialization genes in the right cells at the right time. So the planarian can rebuild itself almost from scratch, whereas the axolotl can rebuild only if the main body axis is intact. This year, researchers took another step toward detailing the molecules underlying regeneration by sequencing the genomes of those two species. The ultimate hope: One day, we'll be able to coax injured humans to execute the same repairs. —Elizabeth Pennisi
Stealing genes to survive
Imagine a raging infection in the lungs of a hospitalized cancer patient. When a powerful antibiotic floods the patient's system, the bacterium responsible, Klebsiella pneumoniae (pictured), seems to be doomed. But it can deploy a resilience strategy honed over billions of years: borrowing a gene from another cell that enables the pathogen to survive.
When environments change, organisms adapt or die. K. pneumoniae and other bacteria have turbocharged the process of adaptation by snagging genes from elsewhere, including various bacteria and DNA molecules loose in the environment. Such horizontal gene transfers allow the bugs to gain valuable new traits, everything from the ability to thrive on cheese rinds to antibiotic resistance.
Researchers think that K. pneumoniae acquired its antibiotic disrupter gene, blaKPC, from another, still-unidentified bacterium. Bacteria outfitted with the gene churn out an enzyme that breaks down several antibiotics.
As with many resilience strategies in nature, stealing genes has its costs. Sometimes microbes incorporate harmful genes instead of helpful ones. And much like a new player on a basketball team, the protein produced from an acquired gene may not mesh with the cell's other proteins. But unfortunately for patients, K. pneumoniae's strategy works all too well: Those bugs kill between 40% and 70% of the people they infect. —Mitch Leslie
Squirrels with a rainy day fund
Scurrying around the South Dakota prairie, 13-lined ground squirrels (pictured) mark the approach of winter by bingeing. By the time a squirrel holes up to hibernate, its weight will have soared by about 40%, thanks to extra fat that will tide the creature over until spring.
During droughts, migrations, bleak winters, and other challenges, organisms often face times when resources are scarce. To get by, the ground squirrel, like many other creatures, stockpiles resources to use later. It can gain more than 2% of its body weight in a single day as it gorges on seeds, grasshoppers, and other delicacies.
But the tactic has downsides. A roly-poly rodent is easier prey for a hawk or coyote. The rainy day fund also can run out prematurely. So once a squirrel is nice and tubby, it enters hibernation, slashing its energy expenditure by 90%. Its body temperature drops to just above freezing and its heart rate falls to as low as 5 beats per minute, down from the usual 350 to 400.
Packing on the fat requires metabolic and behavioral adjustments. But somehow, the squirrel dodges the health problems that plague obese people. Although it develops some of the metabolic defects of type 2 diabetes, the animal isn't sick. And by spring, it is lean and spry and ready to begin the cycle again. —Mitch Leslie
A plant that stands and fights
Unlike those of us on legs, plants can't run away from what they don't like—yet they show remarkable resilience when under attack. Consider how the wild tobacco plant (Nicotiana attenuata, pictured), a meter-high native of North America, protects itself from hungry insects. The plant senses the amino acid compounds in a caterpillar's saliva and responds with an alarm signal—a hydraulic or electrical pulse through its stems and leaves. Within minutes, the plant's cells rev up their production of nicotine, a poison that interferes with an animal's muscle function. When attacked, a single wild tobacco leaf can pack in a half a cigarette carton's worth of nicotine. But some caterpillars, such as hawkmoths, have evolved a way to pass that poison through their gut instead of absorbing it, forcing wild tobacco to unearth new countermeasures. The plant produces compounds that inhibit digestion and make the caterpillar sluggish, as well as abrasives that wear down the attacker's mouthparts. At the same time, the plant calls in help by emitting a scent that attracts ground-dwelling bugs and other caterpillar eaters, and then puts up chemical signposts to guide those predators to their already sluggish prey. Finally, a plant under siege redirects its resources, putting off flowering and growth until the caterpillars are gone. Amazingly, all of this is orchestrated not by a centralized brain, but by decision-making cells scattered throughout the plant. —Elizabeth Pennisi
Fish that switch sex to thrive
Fish are masters of reproductive resilience. About 450 species switch sexes over their lifetimes to maximize their number of offspring. The fish do so by undergoing hormonal changes that transform their organs from those of one sex to the other. Patterns of sex switching vary by species. Big females produce more eggs than little females, so for some species, such as clownfish, it's best to be a male early in life when more runty and then switch to a female later on. But among males that fight each other for females or territories—such as groupers, sea breams (pictured), and porgies—being a too-small male can mean no offspring at all. In that case, it's better to stay a small female instead.
Now, this age-old strategy is allowing fish like the sea bream to adapt to a modern challenge that also disrupts the sex balance: overfishing. Fishers favor the biggest catch. Because one sex is usually bigger than the other, the bigger sex risks being fished out. But researchers have found that sea breams—flavorful, reddish fish common in warmer Atlantic Ocean coastal waters—are ready. Removing big males prompts earlier-than-usual sex changes in some females, so the sex balance is preserved. Still, it's more a short-term strategy than a long-term solution, researchers say. The fish are switching sex at younger ages, so females don't have a chance to grow big. That trend translates into fewer offspring and a shrinking population. That resilience strategy keeps them reproducing for now—but the fish can't save themselves all on their own. —Elizabeth Pennisi